This invention relates to a transmission type screen used as a display screen for projection television, microfilm readers and so on, and to a method of manufacturing this type of screen.
Transmission type screens are widely used as a means for displaying film-advertisements, projected television images, microfilm images, and so on. Ordinarily, this kind of transmission type screen has a certain type of lens disposed on the incidence or viewing surface to make the screen brighter as seen from the observation side and/or to increase the viewing angle. For example, the use of double-side lenticular lenses, fly-eye lenses or the like are disclosed in Japanese Patent Unexamined Publication No. 58-59436, Japanese Utility Model Publication No. 52-4932, Japanese Patent Unexamined Publication Nos. 57-81254, 57-81255 and 58-108523.
To obtain the desired characteristics of these types of screens, the positional relationship between lenticular lenses or the fly-eye lenses on both sides of the screen must be controlled with accuracy. For example, in the case of lenticular lenses having pitches of about 1 mm, there is a need for a degree of positional accuracy such that both extent of misalignment of axes of the two surfaces and variation in the plate thickness are limited to a range of .+-.2%, i.e., to .+-.20 .mu.m. If errors in these factors are not limited to this range, several problems may be encountered, including that of deterioration of color balance, that of a reduction in the visual field range, and that of non-uniformity of colors in the image area.
Almost all double-side lenticular lens type screens put to practical use at present are products formed of methacrylic resins, and the following methods are adopted for forming them:
1 a method of forming an extruded resin sheet with rolls; PA0 2 a casting method based on cell casting; and PA0 3 a compression molding method based on heat pressing. In each method, a pattern of a mother die is transferred directly or indirectly to a resin sheet.
To form a lenticular lenses with accuracy, there is a need for
A high dimensional precision for mother dies for the two lens surfaces;
B evenness of the mold temperature at the time of forming and uniformity of the mold shrinkage of the resin; and
C high accuracy of positioning of the two dies for both surfaces of a screen and prevention of play In a case where a 1 m square double-side lenticular lens type screen having lenses pitches of 1 mm and a thickness of 1 mm is formed while the allowable accuracy range of each of the lateral misalignment and the plate thickness is .+-.2%, the mutual positional accuracy of the two surfaces of the double-side lenticular lens type screen must be maintained so that errors in the factors relating to A, B and C are limited to a range of .+-.20 .mu.m in the lateral direction and in the direction of plate thickness.
However, the linear expansion coefficient of metals are, for example,
______________________________________ steel: 1.1 .times. 10.sup.-5 1/.degree.C. aluminum: 1.7 .times. 10.sup.-5 1/.degree.C. brass 1.8 .times. 10.sup.-5 1/.degree.C. ______________________________________
and if the temperature changes by 1.degree. C., the calculated, expansion contraction of steel per unit length cf 1 m is 11 .mu.m, that of aluminum is 17 .mu.m, and that of brass is 18 .mu.m. Techniques and equipment of an extremely high order are therefore needed for working the molds with improved accuracy (including the accuracy with which the temperature in the factory is controlled), for controlling the forming temperature and for positioning the dies for the two lens surfaces.
Recently, the development of large television display units having finer pixels are promoted, and techniques for forming screen lenticular lenses having smaller pitches are also required. As described above, at the present industrial technology level, it is difficult to remarkably improve the accuracy with which a large lenticular lens type screen, e.g., a 1 m square plastic lenticular lens type screen is formed, i.e., to limit errors in positioning the obverse and reverse surfaces of the lenticular lens type screen to a range narrower remarkably than that mentioned above.
If transparent cylindrical members are used in such a manner that the upper half and the lower half of each cylindrical member are respectively used as lenticular lens units for forming the incidence and viewing surfaces, at least the positional relationship between the lenticular lenses on both surfaces of the screen can be maintained easily. Examples of transmission type screens formed by using such a transparent cylindrical members are disclosed in Japanese Patent Unexamined Publication No. 47-28925, Japanese Utility Model Unexamined Publication Nos. 59-121647, 59-121648, and 59-123850.
As can be immediately recognized in trial manufacture of these examples, any one of these examples entails a problem in that gaps are inevitably formed between the cylindrical members and light passing straight though these gaps disturbs the observation of an image (see-through, hot band and the like take place). In practice, it is impossible to provide screens having good properties.
The variation in the diameter of cylindrical members formed of transparent fibers available at present is at least .+-. several percents, gaps are inevitably formed between such cylindrical members if the cylindrical members are simply arranged. Even if the cylindrical members are arranged while being pressed against each other, the gaps cannot be completely eliminated. That is, the formation of gaps cannot be prevented unless a means for forcibly fixing the cylindrical members is used.
Japanese Patent Unexamined Publication No. 47-28925 discloses a transmission type screen which is constructed in such a manner that lengthwise cylindrical lenses are arranged to constitute a layer one surface of which is coated with a dark color paint while some portions are left uncoated, thereby enabling scattering of emanating light. However, no method is specified for manufacturing this transmission type screen, and this type of product has not been put to practical use in the industry. It is therefore possibly considered that no specific process for this type of lens screens has been established, and that the formation of gaps cannot be eliminated and the leakage of light through the gaps cannot be prevented during use.